The present invention relates to the processes for sulfur recovery from the hydrogen sulfide (H2S) containing gases, using the catalytic conversion of H2S to elemental sulfur on the solid catalysts. In the practice, these processes are exploited for production of commercial sulfur and/or for reduction of the hazardous discharges of sulfur compounds into the atmosphere.
The main contemporary process widely used for sulfur recovery from the H2S-containing gases is the Claus process, named so in honor of Carl Friedrich Claus, a German chemist who in the second half of the XIX century worked in England. He was the first who used a catalytic reaction for conversion of hydrogen sulfide into elemental sulfur by its oxidation with oxygen of the ambient air over the solid catalysts. The catalyst for the process was a bog iron ore. The process was conducted in the adiabatic reactors. In 1883, the British patent office granted him a patent on this process.
The oxidation of hydrogen sulfide to sulfur (Sn) with oxygen proceeds in accordance with thermochemical equation 1:H2S+1/2O2=1/nSn+H2O+ΔH1  [1]
In this equation, “n” is an average number of sulfur atoms in the sulfur molecules formed in the reaction. For the sulfur molecules in a gas phase, this number varies from 1 to 8, depending on the temperature, and when the temperature rises, “n” decreases. At the temperatures lower than 300° C., in the gas phase, only S6 and S8 are thermodynamically stable. In the range of 150-300° C., the average value of “n” is about 7.5.
The ΔH1 is a heat of reaction 1; at 250° C., ΔH1=−49.6 kcal per mole of H2S. Because of this reaction heat, oxidation of one mole of hydrogen sulfide in 100 moles of a gas mixture causes an increase in temperature of the gas mixture of 65-70° C. The free energy ΔG1 (Gibbs energy) of reaction 1 has a big negative value. At 250° C., ΔG1=−41 kcal per mole of H2S. Therefore, this reaction is irreversible, which means that theoretically hydrogen sulfide could be completely converted to sulfur by this reaction.
In the process of the catalytic oxidation of hydrogen sulfide, not only elemental sulfur but also sulfur dioxide (SO2) is formed, according to the following reactions:H2S+3/2O2=SO2+H2O+ΔH2  [2]1/nSn+O2=SO2+ΔH3  [3]At 250° C., ΔH2=−124 kcal per mole of hydrogen sulfide and ΔH3=−75 kcal per mole of sulfur dioxide formed. The reactions 2 and 3 are irreversible because of their big negative Gibbs energies: At 250° C., ΔG2=−113 kcal and ΔG3=−72 kcal per mole of sulfur dioxide formed. At temperatures 290° C. and higher, both of these reactions proceed with fast increasing rates.
Because of the high exothermicity, the Claus method in its original mode (in an adiabatic reactor) is not suitable for treating the gases with the H2S concentration more than 2.0% by volume. For this reason in 1936, I.G. Farbenindustrie A. G. (a German conglomerate of chemical companies) introduced a multi-stage Claus process, which is fit to treat the gases with much higher concentrations of H2S (more than 20% by volume) and which up to now remains a cornerstone of the existing versions of the Claus process. The classic Claus process includes a thermal (non-catalytic) stage and two or three catalytic stages, in which hydrogen sulfide reacts with sulfur dioxide, formed in the thermal stage, in accordance to reaction 4.2H2S+SO23/nSn+2H2O+ΔH4  [4]At 250° C., ΔH4=−24 kcal and ΔG4=−11 kcal per mole of sulfur dioxide. The maximal sulfur recovery in the Claus processes is determined by the thermodynamics of the reversible reaction 4 and cannot exceed 98%. In practice, the sulfur recovery efficiency (SRE) of the Claus processes with two catalytic stages is not more than 95% and with three catalytic stages is about 97%.
At the same time in the USA, the current national emission standards for big sulfur recovery plants is set as low as 250 ppm of sulfur dioxide (on a basis of zero excess air and no moisture), which means that the SRE of the large Claus plants should be at the level of 99.9% and higher. That is why the tail gases of the modern Claus units must be additionally cleaned up before they are discharged into the atmosphere. According to Goar B. G. and Sames J. A. (see Sulfur Recovery, p. 8-3), there are over sixteen commercialized tail gas clean up (TGCU) processes, which are known to those skilled in the art. Many of the TGCU units can comprise the following catalytic stages:                Selective oxidation of hydrogen sulfide to elemental sulfur, e.g. Selectox, SuperClaus, and Hi-Activity processes        Hydrogenation (reduction) of all sulfur compounds, including sulfur dioxide, to hydrogen sulfide, e.g. SCOT (Shell Claus Off-gas Treating) and BSR (Beavon Sulfur Removal) processes        Both hydrogenations of sulfur compounds and consecutive selective oxidation of the formed hydrogen sulfide, e.g. BSR/Selectox, BSR/Hi-Activity, PROClaus, and EuroClaus.The named catalytic stages can be followed with absorption (reversible or irreversible) of SO2, by the aqueous solutions with pH more than 7 (seven). Many TGCU processes are described in the book Gas Purification, by Arthur Kohl et al., in the review Tail Gas Clean-up Processes by Goar B. G. and Sames J. A. (see Sulfur Recovery, pp. 8.3-8.30), and in numerous patents.        
The first commercialized TGCU method using a heterogeneous catalytic oxidation of hydrogen sulfide with oxygen of the air on a solid catalyst, Selectox-32, was developed and tested by Ralph M. Parsons Co. at the end of the 1970s. The catalytic oxidation of H2S was conducted at the second stage of the two-stage BSR/Selectox process, in a two-bed catalytic reactor with a cooling coil positioned between the catalyst beds. In the BSR/Selectox method, the tail gas first passes through the BSR hydrogenation reactor, where supposedly all sulfurous compounds are reduced to hydrogen sulfide over cobalt-molybdate catalyst. Then the gas stream is freed from the most part of water, passing a direct-contact quench tower. The water vapors content decreases to about 5% by volume. The “dried” tail gas is mixed with the air, reheated and enters the Selectox reactor.
Even though the Selectox process is frequently named a direct oxidation of hydrogen sulfide to elemental sulfur, actually in this process H2S is not oxidized directly to sulfur. Actually, in the upper catalyst bed, one third of the hydrogen sulfide is oxidized with the admixed oxygen to sulfur dioxide (reaction 2). In the bottom catalyst bed, the formed sulfur dioxide reacts with the remained amount of hydrogen sulfide (reaction 4). The amount of air admixed to the gas must provide the ratio of H2S/SO2=2, as strict as possible, because this ratio is optimal for the classic Claus reaction. Passing through the cooling coil, the gas is cooled so that the conditions in the bottom catalyst bed become thermodynamically more suitable for the Claus reaction 4. The conversion of hydrogen sulfide into elemental sulfur in the BSR/Selectox process is determined by the equilibrium of reversible reaction 4 and cannot exceed the thermodynamical limit. Therefore, in the Selectox reactor, only about 80% of hydrogen sulfide converts into elemental sulfur. In order to avoid these thermodynamic limits, it is necessary to use the catalysts which are active in the selective oxidation of hydrogen sulfide to elemental sulfur and inactive (or almost inactive) in the Claus reaction.
In the last decades, a number of solid oxide catalysts have been developed for the selective oxidation of hydrogen sulfide to elemental sulfur in gas phase. Many of them are described, for example, in our book Sulfur Compounds of Natural Gases and Oils, pp. 109-126, in Van den Brick's monograph The Selective Oxidation of Hydrogen Sulfide to Elemental Sulfur on Supported Iron-Based Catalysts, in our article Catalytic Oxidation of Hydrogen Sulfide of Natural Gas, and in the following U.S. Pat. Nos. 4,374,819; 4,519,992; 4,576,925; 4,818,740; 4,886,649; 4,988,494; 5,512,260; 5,597,546; 5,603,913; 5,700,440; 5,891,415; 5,965,100; 6,017,507; 6,083,473; 6,099,819; 6,207,127; 6,506,356; 6,919,296.
Among the solid catalysts recommended for the selective oxidation of hydrogen sulfide, the iron-zinc-oxide catalyst, developed and described by us in U.S. Pat. Nos. 5,603,913 and 5,891,415, up to now is the best solid catalyst for oxidation of hydrogen sulfide to elemental sulfur. Now it is known as the Hi-Activity catalyst. It is practically inactive in the reverse Claus reaction; therefore, it can provide complete oxidation of hydrogen sulfide. At full oxidation of hydrogen sulfide, the selectivity of its oxidation to elemental sulfur reaches 96-97%, which is certainly much better than the selectivity of the Selectox, SuperClaus, and other known catalysts.
However, when the Selectox catalyst in the BSR/Selectox unit was replaced with the Hi-Activity catalyst, the following unpredicted facts were observed:                The oxidation of hydrogen sulfide on the Hi-Activity catalyst in the Selectox reactor, which was located after a direct-contact quench tower, started at the temperatures as low as 155-160° C., while in the regular (small) laboratory reactors this happened at much higher temperatures;        If the gas stream before the oxidation reactor bypassed the direct-contact quench tower, the initiation temperature of the hydrogen sulfide oxidation on the Hi-Activity catalyst significantly increased (up to 200° C.);        Cooling the gas stream in the reactor with the cooling coil of the Selectox reactor impeded full oxidation of hydrogen sulfide on the Hi-Activity catalyst in spite of the fact that the temperature in the reactor remained higher than the dew point of sulfur.Such unusual performance of the Hi-Activity catalyst in the Selectox reactor evidenced that some nuances of the process remained unknown and consequently was not taken into account.        
A similar situation has taken place in the SuperClaus process, patented in 1987 by VEG-Gasinstitute N.V. and Comprimo, B.V (U.S. Pat. No. 4,988,494). At the last stage of this process, hydrogen sulfide is selectively oxidized to elemental sulfur on the iron-oxide-containing catalysts. According to Jacobs Comprimo, now about 200 SuperClaus units are used or are in construction worldwide. In all of these units, the concentration of H2S in the inlet gas of the oxidation reactor is 0.8-1.0%, the initiation temperature of the oxidation is about 200° C., and the conversion of H2S into elemental sulfur is at the level of 85%. At the same time, the results of the laboratory study presented by P. J. van den Brink in his monograph “The Selective Oxidation of Hydrogen Sulfide to Elemental Sulfur on Supported Iron-Based Catalysts” shows that the oxidation of hydrogen sulfide on the SuperClaus catalysts starts at the temperatures as low as 160° C., even when the space velocity is as high as 12,000 h−1. These discrepancies between the catalyst performances in the laboratory and industrial units indicate that some unknown nuances of the process have not be taken into account on the transfer from the laboratory experiments to the industrial realization of the SuperClaus process.
In 2000, two new TGTU processes, namely PROClaus and EuroClaus, were presented. In these processes, sulfur dioxide is hydrogenated to hydrogen sulfide on the solid oxide catalysts in the last Claus reactor, concurrently with the Claus reaction.
The first information about PROClaus (Parsons RedOx Claus) process was presented at the Laurence Reid Gas Conditioning Conference in February 2000. The process is now protected by U.S. Pat. No. 6,214,311, which was filed in September 1998 and issued in April 2001. A complex oxide catalyst used in this process, comprising oxides of Fe, Co, Ni, Cr, Mo, Mn, Se, Cu, and Zn on alumina support, is active in both hydrogenation (reduction) of sulfur dioxide and Claus reaction. The catalyst can be used either instead of a common Claus catalyst or in combination with the Claus catalyst immediately following it in the last Claus reactor.
According to the patent, the laboratory study of the PROClaus process was carried out in a quartz isothermal reactor at the temperatures 200-340° C. and space velocity 1,000 h−1. The inlet gas mixtures simulated the effluent gases of the first and second Claus reactors. The first mixture contained 0.92% of SO2, 1.84% of H2S, 33% of H2O, 1.7% of H2, and 0.08% of CO. The second one contained 0.32% of SO2, 0.65% of H2S, 35% of H2O, 1.7% of H2, and 0.08% of CO. It is essential that the used mixtures did not contain elemental sulfur. In the laboratory study, sulfur dioxide was almost completely (95-97%) reduced to elemental sulfur and hydrogen sulfide, so that SO2 concentration in the effluent gas of the reactor imitated the last Claus reactor dropped to 0.03-0.05%. Such a gas could be directed into a catalytic oxidation reactor for the selective oxidation of hydrogen sulfide to elemental sulfur. According to the calculations of the inventor, the process should provide the overall sulfur recovery of the Claus plant equal 99.5%.
However, an attempt to use the PROClaus process in an industrial unit has failed. The sulfur dioxide concentration in the effluent gas of the Claus reactor contained the patented catalyst was as high as it was after the regular Claus reactors. It is clear that in this case some important but unknown peculiarities of the catalytic reaction were not taken into account too. Moreover, they have never been discussed.
The EuroClaus (Extremely Upgraded Reduction Oxidation) process was developed by Jacobs Comprimo Co., Netherlands, and first was described in Sulphur Magazine, No. 270, in October 2000. The process presents a further improvement of the SuperClaus process. Similarly to the Superclaus, in the EuroClaus process, hydrogen sulfide is burned in the reaction furnace of a Claus plant with a substoichiometric amount of air in order to suppress the formation of sulfur dioxide. The small residual amount of sulfur dioxide in the gas stream after the last Claus stage is hydrogenated into H2S on the pre-sulfided oxide catalyst, a thin layer of which is placed in the bottom section of the last Claus reactor. Then, the formed hydrogen sulfide is selectively oxidized to sulfur in the oxidation reactor on the Superclaus catalyst. The calculated overall sulfur recovery efficiency of the EuroClaus process should be up to 99.7%.
However, similarly to PROClaus process, the EuroClaus could not provide the calculated SRE in the industrial units—the concentration of sulfur dioxide after the last Claus reactor with the hydrogenation catalyst remained too high. It is likely that in this case too some unknown details of the hydrogenation reaction were not taken into account in transfer from the laboratory tests to the industrial scale. The inventors of the EuroClaus process later admitted that the conduction of the hydrogenation (reduction) of SO2 in the same reactor with the last Claus stage has no sense. In U.S. Pat. No. 6,800,261 (filed in Aug. 12, 2002 and published in May 10, 2004), they noted that “for extremely high SO2 conversion and minimum loss of sulfur recovery a separate reduction reactor with a preceding condenser is preferred over an integrated reactor, filled with two stacked layers of respectively a Claus catalyst and a reduction catalyst.” The reasons of the observed discrepancy were not discussed.